Please try to provide enough details on each newly posted forum thread.How does your CSS customization look like?What version of Oxygen are you using?If you are using Oxygen 20.1, are you publishing using the DITA+CSS=PDF transformation scenario (DITA Map PDF - based on DITA & CSS (WYSIWYG)) or the one which does HTML5+CSS=PDF DITA Map PDF - based on HTML5 & CSS ?

Using your CSS I tried to produce a PDF using a sample DITA Bookmap we also ship in the Oxygen installation folder (OXYGEN_INSTALL_DIR\samples\dita\it-book\taskbook.ditamap) but I cannot reproduce the problem.Can you put together a small sample DITA map + topics project on which the problem can be reproduced? We have a tech support page on which you can upload a zip file:

I discussed this problem with our main Chemistry engine developer, we will add an internal issue and try to fix this on our side.In the meantime his recommendation is to replace the entire contents of "p-optional-numbering-deep.css" with:

/* All the parts get a part number. Each chapter gets a chapter number. All other topicrefs following a part do not have a number. */ *[class ~= "map/topicref"][is-part] { counter-reset:toc-chapter !important; counter-increment:toc-part;}

/* All other topicrefs that follow a part should not have a number (i.e appendices) */*[class ~= "map/topicref"][is-part] ~ *[class ~= "map/topicref"]:not([is-part]) > *[class ~= "map/topicmeta"]:before{ content: none !important; counter-increment:none;}

/* * Showing the counters before the titles from TOC. */ /**[class ~= "map/topicmeta"] > *[class ~= "topic/navtitle"]:before { display:none !important; /\* In case this CSS is used in cascade with the classic numbering. *\/}*/

We already fixed this issue in the internal development stream, so, if you are willing to test, I can grant you access to the latest nightly build available of a package containing a ready to use customized DITA-OT (it contains also the latest build of Chemistry engine).If you are OK with that, just send an email to support@oxygenxml.com and specifically request us to provide you with the nightly build and I could send you a link and instructions on how to use it.